This invention relates, in general, to a fluid index of refraction sensor, and more particularly, to a sensor for measuring index of refraction of a liquid fuel.
It is becoming increasingly common to use liquid fuel mixtures such as gasoline/alcohol and gasoline/ether for automobiles. This is particularly true in congested urban areas where the use of gasoline mixtures reduces pollutant emissions. Further, gasoline is a non-renewable resource while alcohol and other gasoline substitutes are renewable. Thus, it is expected that the use of liquid fuel mixtures and alternative fuels will increase dramatically in coming years. To provide maximum utility, automobile engines must be able to run on a variety of fuels and fuel mixtures.
One problem encountered in using alternative fuels and fuel mixtures is that to achieve optimum fuel efficiency and minimum pollutants it is necessary to adjust spark timing and fuel-air mixture being delivered to a cylinder depending upon the ratio of alcohol in the alcohol/gasoline mixture. This is particularly true as the percentage of alcohol increases and when pure alcohol is used. To use alternative fuels effectively an engine must be able to re-tune itself each time the fuel mixture changes. Thus, it is important to monitor the fuel mixture constantly so that the fuel-air mixture and spark timing can be adjusted when a new fuel source is used. Further, it is useful if this monitoring can be done electronically so that the data can be easily used by on board computers and electronic fuel injection systems.
It is known that the index of refraction for gasoline changes substantially when alcohol or other gasoline substitutes are added to the gasoline. Is also known that index of refraction of a fluid can be determined by measurement of a critical angle of light reflection from the fluid. Sensors are available which use this critical angle change to indicate the amount of alcohol which is in the gasoline/alcohol mixture. One such sensor is described in U.S. Pat. No. 4,895,444 issued to Miyata et al. This fuel sensor used a transparent window having a light emitting diode and a single light detector mounted underneath the window. The outer surface of the transparent window was exposed to the fuel mixture. Provided that the index of refraction of the window (n.sub.w) was greater than that of the fluid (n.sub.f), all rays of incident light with incidence angles greater than the critical angle will be reflected towards the detector. Thus, depending on the geometry of the sensor and the index of refraction of the fluid, a variable amount of the detector would be exposed to light, generating a signal with an amplitude which was a function of the index of refraction of the fluid. This signal was not, however, a linear function of index of refraction, and was temperature dependent and subject to distortion in a noisy automotive environment.
It would be desirable to have an index of refraction sensor that is not temperature sensitive and provides greater sensitivity and precision. Improved temperature sensitivity and precision has been achieved using a linear detector array rather than a single light detector element. Such a sensor is described in co-pending U.S. Pat. application Ser. No. 491,772 assigned to the same assignee as the present invention. The detector array could be scanned using a simple shift register circuit to produce an output waveform having a pulse width which was proportional to index of refraction of the gasoline/alcohol mixture. Although this method improved precision and accuracy, the detector array produced a relatively low amplitude signal which complicated signal processing in a noisy automotive environment. Also, the linear photo detector array resulted in an unbalanced detector output since the detector elements farthest from the LED source produced a much lower amplitude output than those elements nearest the LED source. It has been found that current amplitude from the linear detector can vary by a factor of more than 300% across the detector.
Accordingly, it is an object of the present invention to provide an index of refraction sensor with improved signal amplitude.
Another object of the present invention is to provide an index of refraction sensor using an arc-shaped photo diode array.
Another object of the present invention is to provide an index of refraction sensor with a photo detector array having a balanced output from each element of the array.
A further object of the present invention to provide an index of refraction sensor having a photo detector array wherein the size of each element of the array is non-uniform.